I love the Planet Hulk story line, but I think it is too early to be introducing this story into their connected movie universe. Hulk (unlike Thor or Guardians of the Galaxy) is more of a earth bound hero (like Spiderman), and while their may be adventures that take him outside the earth I think it is important to establish him more as a hero of earth before launching him into space. So, I say yes please to the Planet Hulk story line, but hoping it will be after they have developed more with him on Earth. I also agree that it would work better if a Villian is responsible for him being sent to another planet. Perhaps if the Avengers are all battling Thanos with the Infinity Gauntlet in Avengers 3 (not 2 as I think it would be too early) he could teleport Hulk to the battle world, and leave the Hulk stranded. Then perhaps he comes to blame the Avengers for not coming to get him and feels like they didn't search for him (even if they did it's about perception) which could lead to an eventual World War Hulk movie. But agree with some people that have said he really should remain the hero at least for a while longer. Also, keep in mind Marvel's plans are not to stop these movies with your simple stereotypical trilogys. They see their shared universe as being similar to James Bond in that it will just keep going. So, with that said they shouldn't rush into using this story in their MCU as they have more then 3 stories in which to introduce it. They should wait until the time feels right instead of trying to shoehorn it in there just because it's a popular story. If anything that could backfire badly if the stories do not feel like a natural progression.